Power harvesting circuit employing a saturable core transformer

ABSTRACT

A power harvesting system employs a saturable core transformer having first and second primary windings and a secondary winding. The first primary winding is a high impedance winding with a large number of turns and the second primary winding is a low impedance winding with a small number of turns. The first and second primary windings are connected to a load. A relay is operable in a first state to connect A/C power to the first primary winding and in a second state to connect A/C power to the second primary winding. When A/C power is connected to the first primary winding, a small current flows in the first primary winding which is insufficient to activate the load but sufficient to transfer power to the secondary winding. When A/C power is connected to the second primary winding, a larger current flows in the second primary winding sufficient to activate the load and to transfer power to the secondary winding.

CROSS REFERENCE TO RELATED APPLICATION

Applicants claim benefit of Provisional Patent Application Ser. No.62/344,493 filed Jun. 2, 2016, by Emilio A. Fernandez and Angel P. Bezosfor “Power Harvesting Circuit”.

BACKGROUND OF THE INVENTION Field of the Invention

The present invention generally relates to a power harvesting circuitand, more particularly, to a power harvesting circuit which providespower to electronic components in an electronic thermostat replacementfor mechanical or mercury bulb type thermostats.

Background Description

Prior art mechanical or mercury bulb type thermostats are used in manyapartment buildings, houses and industrial installations. These devicesdo not use any electrical power to operate and perform their temperaturecontrol function by mechanical means using temperature sensitivemechanical devices that move in order to close an electrical contact,which usually drives directly a load such as a fan motor or othersimilar HVAC device or a relay that then drives such devices. When thesemechanical type thermostats are replaced by modern electronicthermostats, there is a problem of how to power the electronicthermostat. The choices have come down to running an extra wire, whichis usually prohibitively expensive, or devise other means, such asbatteries with which to power the electronic thermostat.

Examples of prior art efforts to solve the problem of providing power toreplacement thermostats include U.S. Pat. No. 4,177,923 to Krump whichdiscloses a battery operated thermostat timer with battery chargingcircuits. Krump uses a transformer with a winding in series with theload to provide charge to the battery and a diode and resistorcombination to provide charge to the battery when the thermostat circuitis open. Since the load is very large in comparison with the power takenin either contact open or contact closed position, then the small amountof power to keep the battery charged is said to be “stolen” or“harvested” from the usual power that is delivered to the load.

Another timer/battery charging system similar but more sophisticatedthan described by Krump is disclosed by U.S. Pat. No. 4,249,696 toDonnelly et al. This circuit relies on a Triac and an accompanyingcontrolled solid state and gating means.

U.S. Pat. No. 4,333,605 to Peters describes a high impedance/lowimpedance power supply for a temperature control system. According toPeters, the temperature is controlled by turning on or off anelectromagnetic relay which then turns on or off a motor or anotherrelatively high power load (compared to the relay), such as a resistanceheater. The electromagnetic relay provides the power supply with aconstant load with known characteristics and with relatively lower powerdemand when compared with the load driven by the relay contacts.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide a simpleand relatively inexpensive power harvesting circuit for powering areplacement electronic thermostat.

It is another object of the invention to provide a power harvestingcircuit for powering an electronic thermostat which is designed to powera large, low impedance high current load, such as a ¼ HP or ½ HP, 115VAC motor directly with minimum power dissipation in the harvestingcircuit.

Minimizing power dissipation is critical since the space available forthe power harvesting circuit and the electronic thermostat that itpowers is very small, ideally the inside of a standard electricaljunction box, and the power dissipation can have an effect and corruptthermostat readings. This is so important that Johnstone et al. in U.S.Pat. No. 4,776,514 use a thermistor to monitor the temperature increasedue to power dissipation and uses this information to compensate thethermostat readings. Of course, using a thermister increases complexityand adversely affects costs.

It is another object of the invention to handle variable loads such as ⅛HP to ½ HP 115 VAC motors that might be installed in the HVAC system.

Being able to handle varying loads is a significant advantage as alarger number of installations can be handled by the same device andalso because, due to maintenance and other factors, the load can changeduring the life of the product. The ability to work with variable loadsis important if not critical when retrofitting existing systems with newelectronic thermostats as the size of the motor (load) can vary and isoften not known in advance. Also due to the fact that motors have afinite life and are routinely changed, sometimes with a motor thatpresents a different load to the system.

Also, the commercial realities are that this type of thermostat has tobe inexpensive and very reliable and maintenance free as maintenancecalls are very expensive both in terms of dollars and in terms ofcustomer inconvenience or dissatisfaction if the thermostat is used in ahabitable space such as an apartment.

According to the invention, there is provided a saturable coretransformer having two primary windings and a secondary winding. One ofthe primary windings is a high impedance winding, and the other primarywinding is a low impedance winding. The two primary windings areconnected with the load (motor). The secondary winding provides power tothe circuit components of the electronic thermostat. Relay contactsconnect A/C power to the high impedance primary winding in a first stateor to the low impedance primary winding in a second state. When therelay is de-energized, A/C power is applied to the high impedancewinding so that a relatively small amount of current flows through thehigh impedance winding. This current is low enough that it does notenergize the motor and in fact is invisible to the motor and does notaffect motor operation at all but is sufficient to generate the requiredvoltage to transfer power to the secondary winding to power theelectronic thermostat. When the relay is energized, A/C power is applieddirectly to the low impedance primary winding, a larger current flows inthe low impedance primary winding energizing the motor. As the currentthrough the low impedance winding builds up, the core saturates. Theresult is that a relatively short pulse is generated in the secondary onboth the positive and negative A/C cycle. This pulse has an amplitudedetermined by the turns ratio of the low impedance winding to thesecondary winding and is used to power the electronic thermostat. Afterthe core saturates, the impedance of the low impedance winding is onlythe resistance of the wire of the winding which is very small andresults in negligible impact on the motor operation and also results inrelatively low power dissipation compared to the load.

In a first embodiment of the invention, the first and second primarywindings are connected in series with the load so that when the relayconnects A/C power to the first contact, current flows through both thefirst and second primary windings of the transformer, and when the relayconnects A/C power to the second contact, current flows only in thesecond primary winding. In a second embodiment of the invention, thefirst and second primary windings are separately confected with the loadso that when the relay connects A/C power to the first contact, currentflows through only the first primary winding of the transformer, andwhen the relay connects A/C power to the second contact, current flowsonly in the second primary winding.

In a modification to the first and second embodiments, an approximationof the current flowing through the second primary winding of thetransformer is sensed and this information is used to protect thecircuit from a motor short. When the sensed current exceeds apredetermined threshold, the relay is actuated to remover the motorload.

According to a third embodiment of the invention, one or more switchescan be controlled to selectively short a predetermined number ofwindings of the transformer second primary winding in response to thevoltage output of the unregulated power supply exceeding one or morepredetermined thresholds. The shorting of the second primary windingsresults in the output voltage of the power supply being decreased inproportion to the number of windings shorted. The fourth embodiment is avariation of the third embodiment, but instead of shoring only a portionof the second primary winding, the whole second primary winding isshorted when the voltage output of the unregulated power supply exceedsthe predetermined threshold. After the voltage of the unregulated powersupply reaches a predetermined value, the switch will turn off,unshorting the second primary winding. The advantage of this approach isthat it does not require multiple taps or multiple solid state switches.The disadvantage is that it requires faster switching times of the solidstate switches and faster microcontroller response, thus increasing costand complexity. To achieve faster solid state switches, a DC/DCconverter is used to power a different and faster type of solid stateswitch.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other objects, aspects and advantages will be betterunderstood from the following detailed description of a preferredembodiment of the invention with reference to the drawings, in which:

FIG. 1 is a schematic diagram of a first embodiment the power harvestingcircuit according to the invention;

FIG. 2 is a schematic diagram of a second embodiment of the powerharvesting circuit according to the invention;

FIG. 2A is a schematic diagram of an improvement to the first and secondembodiments of the power harvesting circuit according to the intention;

FIG. 3A is a waveform diagram illustrating the current waveform whencurrent flows in the high impedance winding of the power harvestingcircuit and the core of the transformer is unsaturated;

FIG. 3B is a waveform diagram illustrating the current flow in thesecondary winding when current flows in the low impedance winding of thepower harvesting circuit and the core of the transformer is saturated;

FIG. 4 is a simplified circuit diagram of the power supply for theelectronic thermostat;

FIG. 5 is a schematic diagram of a third embodiment of the powerharvesting circuit according to the invention;

FIG. 6 is a schematic diagram of a fourth embodiment of the powerharvesting circuit according to the invention; and

FIG. 7 is an alternative circuit diagram of a circuit for keeping theunregulated voltage within predetermined limits.

DETAILED DESCRIPTION THE INVENTION

Referring now to the drawings, and more particularly to FIG. 1, there isshown a saturable core transformer 10 having two primary windings 11 and12 and a single secondary win ding 13. Primary winding 11 is a highimpedance winding having, for example, 2500 turns, and primary winding12 is a low impedance winding having, for example, twelve turns. Thesecondary winding 13 has, in this example, 256 turns The two primarywindings are connected in series with the load (motor) 14. The secondarywinding 13 provides power the power supply 15 which, in turn, powerscircuit components of the electronic thermostat. The electronicthermostat typically includes a microcontroller 16 that receives aninput from a Thermister circuit 17 and controls relay contacts K1.Double throw relay contacts K1 connects A/C power to either the highimpedance primary winding 11 or to the low impedance primary winding 12.Although an electro-mechanical relay is illustrated, those skilled inthe art will recognize that the function of this relay can be performedby an equivalent solid-state drive. In some applications, themicrocontroller 16 may communicate with a graphical display 18, whichmay be a touch screen display to provide user input. The microcontroller16 may also communicate with an RF transceiver 19 to provide remotecontrol or remote monitoring of important parameters such as settemperature and amount of time that the relay, and consequently themotor, are energized and de-energized. The transceiver 19 may be, forexample, a WiFi or Bluetooth transceiver for local communication or ahigher powered transceiver for distant communication.

When the relay K1 is de-energized, A/C power is applied to the highimpedance winding 11 so that a relatively small amount of current, onthe order of 10 to 15 mA, flows through both the high impedance winding11 and the low impedance winding 12. This current is low enough that itdoes not energize the motor 14 and has negligible impact on the lowvoltage coil, but it is sufficient to generate the required voltage totransfer power to the secondary winding 13 and is used to power theelectronic thermostat. When the relay K1 is energized, A/C power isapplied directly to the low impedance primary winding, energizing themotor 14. At the beginning of each A/C cycle, the current through thelow impedance winding builds up rapidly until the core of transformer 10saturates. The result is that a relatively short pulse (about 2milliseconds) is generated in the secondary winding 13 on both thepositive and negative A/C cycle. This pulse has a current amplitudedetermined by the turns ratio of the low impedance winding to thesecondary winding. This pulse is filtered and processed by the powersupply 15 to power the electronic thermostat. After the core saturates,the impedance of the low impedance winding 12 is only the resistance ofthe wire of the winding which is very small. The voltage drop acrossprimary winding 12 is very small, on the order of 1 to 2V peak and 0.7to 1.4V RMS, which has a negligible impact on the motor operation.

FIG. 2 shows an alternate embodiment in which the two primary windings11 and 12 are alternately connected to the A/C power with currentflowing on only one or the other of the two primary windings. When thecontacts of relay K2 are open, A/C current flows in only the highimpedance winding 11. As a result, the current flowing through the highimpedance winding is low enough that it does not energize the motor 14but is sufficient to generate the required voltage to transfer power tothe secondary winding 13 and is used to power the electronic thermostatas in the first embodiment. However, when the contacts of relay K2 areclosed, A/C power is supplied directly to the low impedance winding 12and, after the core of transformer 10 saturates, the impedance of thelow impedance winding 12 is only the resistance of the wire of thewinding which is very small. Since the impedance of this winding is verylow, there is negligible effect on the operation of the motor 10.

FIG. 3A is a waveform diagram showing the current flow in the secondarywinding 13 when relay contacts K1 supply A/C current to the highimpedance winding 11 of the power harvesting circuit of FIG. 1 or therelay contacts K2 supplying A/C current only to the high impedancewinding 11 of the power harvesting circuit of FIG. 2. Notice that thiscurrent waveform is substantially a sinusoidal waveform having anamplitude determined by the turns ratio of primary winding 11 tosecondary winding 13.

FIG. 3B is a waveform diagram showing the current flow in the secondarywinding 13 when relay contacts K1 supply A/C current only to the lowimpedance winding 12 of the power harvesting circuit of FIG. 1 or whenrelay contacts K2 supply A/C current directly to low impedance winding12 bypassing the high impedance winding 11 of the power harvestingcircuit of FIG. 2. In both these cases, the current waveform is a seriesof positive and negative going pulses due to the saturation of the coreof the transformer.

From the foregoing, it will be appreciated that the circuits of FIGS. 1and 2 are able to handle varying loads since these circuits have minimaleffect on the A/C current delivered to the load since the impedance ofthe saturable core transformer at saturation is essentially theresistance of the wire. Also, because the saturation characteristics ofthe transformer, the increase in the current transfer to the secondarywith increasing current in the primary is minimized. This is asignificant advantage as a larger number of installations can be handledby the same device and also because, due to maintenance and otherfactors, the load can change during the life of the product. The abilityto work with variable loads is important if not critical whenretrofitting existing systems with new electronic thermostats as thesize of the motor (load) can vary and is not always known in advance andalso due to the fact that motors have a finite live and are routinelychanged, sometimes with a motor that presents a different load to thesystem.

FIG. 2A illustrates a modification of the FIG. 2 circuit. Themodification consists of the addition of a scaling circuit 20 that feedsan A/D converter in the microcontroller 16. The voltage fed to themicrocontroller A/D is the scaled down unregulated power supply voltageV_(UR). This voltage is roughly proportional to the current flowingthrough the primary of the transformer 10. Thus the microcontroller hasan approximate analog to the current flowing in the primary of thetransformer and this information is used to protect the circuit from amotor short and thus would make it possible to eliminate the fuse (notshown) currently used. This eliminates a problem in that momentaryshorts in the motor blow the fuse and cause an equipment failure.Momentary motor shorts can occur when the load driven by the motor,which is usually a fan, gets momentarily mechanically stuck due tonormal wear and tear or due to some momentary mechanical irregularity.With the microcontroller having a reference to the approximate currentflow, an upper limit can be set at which the microcontroller willactuate the relay and remove the motor load. If this happens a fewtimes, then the microcontroller can provide this information tomanagement so that corrective action can be taken. And more importantly,a thermostat failure (due to a blown fuse) and subsequent costly servicecall has been prevented. This might not appear that critical but thosefamiliar with the commercial realities of this application will considerthis feature indeed a necessity.

FIG. 4 is a simplified circuit diagram of the power supply which iscapable of rectifying, filtering and regulating the output of thesecondary winding of either the first or the second embodiments of thepower harvesting circuit according to the invention. Specifically, thesecondary winding 13 of the power transformer 10 is split into twosecondary windings 41 and 42 which are connected to a rectifier bridge43. This rectifier bridge 43 is connected to a clamp circuit 44 and thento a low pass filter 45 composed, for example, of a series of parallelcapacitors. The output of the low pass filter 45 is connected to aregulator 46, which may be, for example, a step-down switchingregulator. The purpose of the clamp 44 is to limit surges or voltagespikes that might damage the regulator 46. These voltage surges might becaused by a malfunctioning motor drawing excessive current. The outputof the regulator 46 is supplied to the electronic thermostat.

For power dissipation reasons, the power supply load for this thermostatapplication has to be a switching type power supply, as a pass typeregulator would dissipate too much power and, as explained before, powerdissipation in this application is a big concern. As the unregulatedvoltage from the transformer increases due to higher primary currentwhen the low impedance winding is used, this switching power supply loadacts as a negative resistance. This plus the fact that even insaturation the transformer output voltage will increase somewhat withincreasing current limits the practical limit of the current range.Another factor that limits the upper limit of the current range is thatsince size is critical in this application the gauge of wire that onecan use for the primary coil is limited as thicker wire occupies toomuch space. Thus with the maximum size of coil wire that can be usedbecause of size limitations, power dissipation at upper end of thecurrent range becomes an issue.

FIG. 5 shows a further modification of the basic circuit which expandsthe practical primary current range. In this implementation the powersupply unregulated voltage Vur is monitored by the microcontroller. Themicrocontroller 16 is programmed so that if the voltage reaches acertain high threshold, say 25 VDC, the microcontroller will turn “on”transistor Q2 which is coupled through pbotocoupler 51 to turn on FETswitch 52 which in turn shorts out a certain amount, say ½ of the inputcoil of the second primary winding 12. This causes the power supply 15unregulated voltage to decrease by the proportion of coils shorted, inthis example roughly by ½ or down to about 12.5V. Thus, by the use ofthis technique the amount of primary current that can be handled hasessentially being doubled. Note that there could be more primary coiltaps and more FET switches driven by the microcontroller, so thistechnique can be repeated and thus the primary current can be increasedfurther as desired.

FIG. 6 illustrates a further modification of the basic circuit whichoperates fundamentally essentially the same as the circuit in FIG. 5.The only difference is that the FET switch function is accomplisheddifferently and the FET switch is shown shorting the whole secondprimary winding rather than a portion of the winding. More particularly,the output of the photocoupler 51 is input to an FET gate driver 61which is powered by a milliwatt DC/DC converter 62. The FET gate drivercontrols the FET switch 63 which, when turned on, shorts the entiresecond primary winding of the transformer. The main advantage ofperforming the FET switch function this way is that in thisimplementation the switching can be accomplished faster than the circuitof FIG. 5. Faster switching allows the shorting of the whole coil, thusthe microcontroller will regulate the power supply unregulated voltageby switching the FET Switch “on” when a certain maximum threshold isreached (say 25V) and will turn it “off” when the power supplyunregulated voltage reaches a certain minimum threshold, say 12.5V. Theadvantage of this approach is that no transformer taps are needed andthat it is able to accomplish a wide input current range, this rangedepending only on how fast the Microcontroller can regulate the powersupply unregulated voltage. Also, increasing the power supplycapacitance will permit slower microcontroller operation of thisregulating scheme. There are many trade-offs here as those skilled inthe art will appreciate. As an example, a dedicated microcontrollercould be used for this function only, thus reducing any time delays andallowing faster operation.

In the embodiment shown in FIG. 6, the transformer 10 does notnecessarily have to be a saturable core transformer. If both themicrocontroller 16 and the solid state switches are fast enough, thecircuit can operate fast enough to synch with the AC line and either cutwhole cycles “on” and “off” or cutting a portion of each positive ornegative cycles “off” as needed.

The embodiments of the invention described thus far employ onetransformer with two primary windings and one secondary winding. The twoprimary windings are made up of one high impedance high voltage (i.e.,115 VAC) winding and the other one the very low impedance, very lowvoltage winding that is in series with the load. This is the preferredembodiment. However, one could split these two functions into twoseparate modules. One will be a transformer with the low voltage, lowimpedance winding only. This will provide power when the load is “on”and this winding is in series with the load. Then to provide power whenthe load is “off” one could use a separate transformer and separatemodule dedicated to this task. This could be a very high frequencytransformer and power supply, similar to the “cubes” that one can buy topower smart phones. It will work from the 115 VAC but, as it is donewith the smart phone chargers, the 115 VAC is chopped a very highfrequency. This makes the transformer and the whole module small.

The circuit of FIG. 6 shows the microcontroller 16 and an A/D converterthat is part of the microcontroller controlling the power Supply inorder the keep the unregulated voltage within certain limits, forinstance 12 VDC and 24 VDC. The circuit of FIG. 7 performs the samefunction using discrete electronic components. This circuit could beused as back up for the microcontroller 16 in which case the discretecomponent circuit will have wider voltage limits that the Micro. Thiswould provide a safety measure in case that the microcontroller becomesdisabled for any reason, including a software bug. Or themicrocontroller could be used as the backup letting the circuit performthe regulation function. And in cases where there is no Micro, thecircuit alone could perform the unregulated voltage control function.Note that transistor Q1 in FIG. 7 is the same transistor Q2 as in FIG.6.

Under normal operation transistors Q2 and Q3 are either both “on” orboth “off”. Assuming that unregulated voltage Vur is below the highlimit, approximately 26 VCD and that both transistors are “off”, thisstate will remain until Vur increases to approximately 26 VDC. At thispoint and for any voltages higher than this threshold diode CR1 startsconducting and as the voltage increases the current flowing throughdiode CR1 is enough to turn “on” transistor Q2. When transistor Q2 isturned “on”, this in turn causes current to flow through resistor R4which turns “on” transistor Q3. And when transistor Q3 turns “on” itsupplies current to the base of transistor Q2 via diode CR2 and resistorR5, thus keeping transistor Q2 “on” regardless of the amount of currentbeing supplied by diode CR1. Thus transistor Q2 is latched “on” and itremains “on” until the unregulated voltage Vur decreases to the lowthreshold level of approximately 12 VDC. At this point there is nocurrent flowing through diode CR1 and Vur is low enough that there isnot enough current flowing through diode CR2 to keep transistor Q2 “on”.Thus transistor Q2 turns “off” which turns “off” transistor Q3 untilagain Vur increases above the high threshold of approximately 26 VDC andthe process repeats itself in this fashion. The net result is that Vuris thus kept between the low and high thresholds via transistor Q1,which performs the same function in FIG. 7 as it does in FIG. 6. Notethat in FIG. 6, this transistor is labeled Q2. On input to transistorQ1, Vm is provided so that either the microcontroller or the circuit candrive transistor Q1. Thus, either the circuit or the microcontroller canperform the primary regulation function with the other one being thebackup.

While FIG. 7 shows a specific discrete component circuit implementationto accomplish the desired result, those skilled in the art willappreciate that there are many variations of the same circuit conceptpossible using other components like FETs or SCRs or IntegratedCircuits.

While the invention has been described in terms of preferredembodiments, those skilled in the art will recognize that the inventioncan be practiced with modification within the spirit and scope of theappended claims.

The invention claimed is:
 1. A power harvesting system comprising: atransformer having first and second primary windings and a secondarywinding, the first primary winding being a high impedance winding with alarge number of turns and the second primary winding being a lowimpedance winding with a small number of turns, the first and secondprimary windings connected with a load; and a relay having first andsecond contacts, the first contact being connected to the first primarywinding and the second contact being connected to the second primarywinding, the relay being operable in a first state to connect A/C powerto the first primary winding and in a second state to connect A/C powerthe second primary winding, whereby when A/C power is connected to thefirst primary winding, a small current flows in the first primarywinding which is insufficient to activate the load but sufficient totransfer sufficient power to the secondary winding, and when A/C poweris connected to the second primary winding, a larger current flows inthe second primary winding sufficient to activate the load and totransfer sufficient power to the secondary winding.
 2. The powerharvesting system of claim 1, wherein the first and second primarywindings are connected in series with the load so that when the relayconnects A/C power to the first contact, current flows through both thefirst and second primary windings of the transformer, and when the relayconnects A/C power to the second contact, current flows only in thesecond primary winding.
 3. The power harvesting system of claim 1,wherein the first and second primary windings are separately connectedwith the load so that when the relay connects A/C power to the firstcontact, current flows through only the first primary winding of thetransformer, and when the relay connects A/C power to the secondcontact, current flows only in the second primary winding.
 4. The powerharvesting system of claim 1, wherein the relay is a solid state relay.5. The power harvesting system of claim 3, further comparing: a powersupply connected to the secondary winding of the transformer, the powersupply providing an unregulated output voltage; and means for sensingthe unregulated output voltage as an approximate function of currentflowing in a primary winding of the transformer to the load, said meansfurther disconnecting the load when current flowing in the primarywinding exceeds a predetermined value.
 6. The power harvesting system ofclaim 3, wherein the second primary winding of the transformer has oneor more taps and further comprising: a switch controlled to selectivelyshort said one or more taps of the second primary winding; a powersupply connected to the secondary winding of the transformer, the powersupply providing at least an unregulated output voltage, and means forsensing the unregulated output voltage as an approximate function ofcurrent flowing in a primary winding of the transformer to the load,said means further controlling said switch to short said one or moretaps of the second primary winding.
 7. The power harvesting system ofclaim 3, further comprising: a switch controlled to selectively shortsaid second primary winding of the transformer; a power supply connectedto the secondary winding of the transformer producing relatively shortpulses, the power supply providing an unregulated output voltage, andmeans for sensing the unregulated output voltage as an approximatefunction of current flowing in a primary winding of the transformer tothe load, said means further controlling said switch to short saidsecond primary winding.
 8. The power harvesting system of claim 1,wherein said at least one transformer is a saturable core transformerand when a larger current flows in the second primary winding sufficientto activate the load, the larger current causes the core of thetransformer to saturate producing relatively short pulses on thesecondary winding on each cycle of A/C power.
 9. A replacementelectronic thermostat for replacing a mechanical or mercury bulb typethermostat comprising: a saturable core transformer having first andsecond primary windings and a secondary winding, the first primarywinding being a high impedance winding with a large number of turns andthe second primary winding being a low impedance winding with a smallnumber of turns, the first and second primary windings connected with aload; a relay having first and second contacts, the first contact beingconnected to the first primary winding and the second contact beingconnected to the second primary winding, the relay being operable in afirst state to connect A/C power to the first primary winding and in asecond state to connect A/C power the second primary winding, wherebywhen A/C power is connected to the first primary winding, a smallcurrent flows in the first primary winding which is insufficient toactivate the load but sufficient to transfer power to the secondarywinding, and when A/C power is connected to the second primary winding,a larger current flows in the second primary winding sufficient toactivate the load and transfer sufficient power to the secondarywinding; a power supply for the electronic thermostat connected to thesecondary winding of the saturable core transformer, said power supplyrectifying, filtering and regulating current supplied by the secondarywinding of the saturable core transformer; and wherein the electronicthermostat is connected to be powered by said power supply.
 10. Thereplacement electronic thermostat of claim 9, wherein the first andsecond primary windings of the saturable core transformer are connectedin series with the load so that when the relay connects A/C power to thefirst contact, current flows through both the first and second primarywindings of the saturable core transformer, and when the relay connectsA/C power to the second contact, current flows only in the secondprimary winding.
 11. The replacement electronic thermostat of claim 9,wherein the first and second primary windings of the saturable coretransformer are separately connected with the load so that when therelay connects A/C power to the first contact, current flows throughonly the first primary winding of the saturable core transformer, andwhen the relay connects A/C power to the second contact, current flowsonly in the second primary winding.
 12. The replacement electronicthermostat of claim 9, wherein the relay is a solid state relay.
 13. Thereplacement electronic thermostat of 9, wherein the electronicthermostat includes a microcontroller programmed to control functions ofthe electronic thermostat.
 14. The replacement electronic thermostat ofclaim 13, wherein the power supply provides an unregulated outputvoltage, and wherein said microcontroller is connected to sense theunregulated output voltage as an approximate function of current flowingin a primary winding of the saturable core transformer to the load, saidmicrocontroller further disconnecting the load when current flowing inthe primary winding exceeds a predetermined value.
 15. The replacementelectronic thermostat of claim 13, wherein the second primary winding ofthe saturable core transformer has one or more taps, the power supplyprovides an unregulated output voltage, and wherein said microcontrolleris connected to sense the unregulated output voltage as an approximatefunction of current flowing in a primary winding of the saturable coretransformer to the load, further comprising a switch controlled toselectively short said one or more taps of the second primary winding,said microcontroller further controlling said switch to short said oneor more taps of the second primary winding.
 16. The replacementelectronic thermostat of claim 13, wherein the power supply supplies anunregulated output voltage, and wherein said microcontroller isconnected to sense the unregulated output voltage as an approximatefunction of current flowing in a primary winding of the saturable coretransformer to the load, further comprising a switch controlled toselectively short said second primary winding of the saturable coretransformer, said microcontroller is further controlling said switch toshort said second primary winding.
 17. The replacement electronic thenthermostat of claim 13, further comprising a graphical displaycontrolled by said microcontroller to provide user input.
 18. Thereplacement electronic thermostat of claim 13, further comprising an RFtransceiver in communication with said microcontroller to provide remotecontrol.
 19. The replacement electronic thermostat of claim 9, whereinsaid at least one transformer is a saturable core transformer and when alarger current flows in the second primary winding sufficient toactivate the load, the larger current causes the core of the saturablecore transformer to saturate producing relatively short pulses on thesecondary winding on each cycle of A/C power.